1. Field of the Invention
The present invention generally relates to a driver for driving a light-emitting device and an image forming apparatus, and more particularly, to a light-emitting device driver that is suitably used for driving a laser device as a light source of laser xerography and an image forming apparatus that is equipped with the light-emitting device driver.
2. Description of the Related Art
In the field of laser xerography in which laser devices are used as light sources, there is an increasing demand for devices with higher resolution and higher-speed performance. However, there is a limit to the speed at which on/off control can be performed on the driving of a laser device in accordance with input image data (the speed will be hereinafter referred to as the modulation speed). If the resolution in the sub scanning direction, as well as in the main scanning direction, is to be increased in a case where the number of laser beams is one, the modulation speed is also increased. So as to increase the resolution in the sub scanning direction without an increase in the modulation speed, it is necessary to increase the number of laser beams. In a case where the number of laser beams is four and the modulation speed is the same as the modulation speed in the case of one laser beam, the resolution in the main scanning direction and the resolution in the sub scanning direction can be doubled.
Semiconductor lasers that are employed as light sources in laser xerography can be categorized roughly into two types. One of the types is an edge-emitting type (hereinafter referred to as edge-emitting lasers) with a structure for emitting laser beams in a direction parallel to an active layer. The other is a surface-emitting type (hereinafter referred to as surface-emitting lasers) with a structure for emitting laser beams in a direction perpendicular to an active layer. Conventionally, edge-emitting lasers have been used as laser beam sources in laser xerography.
With the edge-emitting laser, however, it is technically difficult to increase the number of laser beams. The surface-emitting laser is more advantageous with an increased number of laser beams than the edge-emitting laser. For this reason, devices equipped with surface-emitting lasers that can emit a large number of laser beams have been developed as the laser beam sources in the field of laser xerography, so as to respond to the public demand for higher resolution and higher-speed performance.
Meanwhile, the driving circuits of laser emitting devices for laser xerography have wiring loads, resistances, and parasitic capacitances that vary with the types of mounting of the laser emitting devices. Therefore, the conventional light-emitting device is equipped with compensating circuit that increases the response of the light-emitting device.
Japanese Unexamined Patent Publication Nos. 62-62572, 9-83442, and 10-284783 (hereinafter referred to as Patent Documents 1, 2, and 3, respectively) each discloses a method for compensating for the driving current of a light-emitting device by transitionally causing current to flow in and out, using a differential current circuit that includes a capacitor C and a resistance R.
Also, driving methods by which voltage driving and current driving are both employed have been developed. Japanese Unexamined Patent Publication No. 5-198844 (Patent Document 4) discloses a technique for preventing micro-light emission by applying a reversed bias voltage when the light-emitting device is not emitting light. Further, Japanese Patent Publication No. 3068723 (Patent Document 5) discloses a technique for preventing excessive current flow by performing a constant voltage driving operation until the current flowing forward in a semiconductor laser reaches a predetermined set value, and then performing a current driving operation.
In each of the techniques disclosed in Patent Documents 1, 2, and 3, the respective compensating circuit has a large circuit size, because a current flows in (or is applied) transitionally in accordance with the CR time constant. When any of these techniques is applied to a multi-laser device that has surface-emitting lasers as light sources, it is necessary to employ compensating circuits that have different time constants suitable for the respective laser beam sources. As a result, the chip size increases, and it becomes difficult to put driving circuits into one chip, especially in a multi-laser device.
Also, by the technique disclosed in Patent Document 4, current and voltage are repeatedly applied in synchronization with image data. So as to prevent micro-light emission in such a structure, it is necessary to constantly apply a voltage while the light-emitting device is not emitting light. Therefore, this structure can neither function as an alternative device for the above-described compensating circuit, nor adjust the voltage-applied time. Furthermore, by the technique disclosed in Patent Document 5, the voltage driving time is determined by the current value. Therefore, the voltage driving time cannot be adjusted in accordance with the performance of each light-emitting device, and this technique cannot provide an alternative mechanism for the above-described compensating circuit.